The present invention relates to devices for reading encoded digital information recorded at high densities as bivalent states of a moving recording medium, and more particularly to networks that function to reduce cross talk between the bivalent pulses of a read signal in order to correct the peak phase shift and peak amplitude distortion that characterizes intersymbol cross talk, thereby facilitating the accurate decoding and retrieval of the information.
Digital information is commonly stored in the form of a bivalent state of a moving magnetic recording medium such as a floppy disc, disc or magnetic drum. The information is typically written onto the recording medium in accordance with either of two encoding techniques, the so-called single density (standard) and double density (self-clocking) recording formats. In the case of the single density recording format, each data bit is recorded onto the recording medium along with an associated clock bit, with the clock bit providing bit separation and facilitating data retrieval. The requirement of a clock bit for each data bit can be eliminated by using a high density self-clocking recording code. Current high density recording codes permit digital data to be encoded and written onto a recording medium at typically double the density of the single density recording format, i.e. the bit cell interval in double density recording is half the duration of the single density bit cell. As conventionally used in the computer arts, a bit cell is a binary information element occupying a fixed interval of time over which a digital bit can be recorded as a bivalent magnetic flux state of a moving recording medium.
The high density codes used to encode digital information are able to designate sequences of digital information in terms of at most one transition, i.e., one change in the magnatic flux state of the recording medium, per bit cell. For example, in one commonly used self-clocking code modified frequency modulation (MFM) a logic one is represented by a transition in the center of a bit cell, while a logic 0 is represented by a transition at the beginning (i.e., the leading edge) of a bit cell unless the immediately preceeding information was a logic 1, in which case the transition is omitted. Thus, any sequence of digital information can be represented in terms of step transitions at intervals corresponding to either 1, 11/2, or 2 bit cells. Referring to FIG. 1, a sequence of double density bit cells (each, for example, two microseconds in duration) is shown on line A, while waveform B represents the indicated digital information in terms of a sequence of step transitions at intervals prescribed by the MFM encoding technique. The digital information represented in wavefore B is designated not only by the presence of a transition (as in the single density recording format), but also by the precise timing of the transitions. Accordingly, in order to retrieve information recorded in a double density format the reading process must preserve the intervals between transitions so that the encoded transition sequence can be recognized thereby enabling the data clock to be recovered.
The use of a double density recording format for encoding digital data does not present a problem from the standpoint of writing the information onto the recording medium. The digital data can be straightforwardly written onto the recording medium by applying to a magnetic head a waveform (such as waveform B) that is substantially rectangular, i.e., one having abrupt step transitions at precisely controlled intervals, thereby causing sudden reversals of the magnetic condition of the recording medium according to the same precise timing sequence. Considerable difficulty, however, is encountered in accurately retrieving the timing sequence of the recording code transitions. As noted above, reading double density encoded data requires that the precise intervals between the transitions, and not just the occurance of the transitions, be detected. Thus, referring to FIG. 1, the information encoded in waveform B can be accurately retrieved only if the read circuitry can detect transitions occuring at times t.sub.0, t.sub.1 (after an interval of approximately two microseconds), t.sub.2 (after an interval of approximately two microseconds), t.sub.3 (after an interval of approximately four microseconds), t.sub.4 (after an interval of approximately three microseconds), and so on.
The difficulty in accurately detecting the precise sequence of the recording code transitions arises because, unlike the write signal waveform used to record the data, the read signal waveform is not at all rectangular. Rather, due to the transfer characteristics of the recording medium and the magnetic head, each transition is translated during the write/read process into an amplitude variation having a Gaussian shape. Thus, the read signal comprises a sequence of elementary Gaussian-shaped transition pulses, the polarities of which alternate, with the information being contained in the phase of the peaks of the transition pulses. Such a sequence of bivalent transition pulses is represented by waveform C in FIG. 1. In the case of data recorded in a double density format, an effect known as pulse crowding causes intersymbol cross talk. That is, because the Gaussian-shaped transition pulses succeed each other at smaller intervals, their superposition in time causes the peak amplitudes of the transition pulses to be reduced (see waveform C). Furthermore, since a transition may occur nearer to the adjacent transition on one side or the other, the superposition of adjacent symmetrical Gaussian waveforms can be nonsymmetrical, with the result that the peak of a transition pulse is shifted in the direction of the larger of the adjacent intervals. Thus, the transition pulse peak in the read signal waveform C that should occur at time t.sub.2 is shifted to the right, away from the preceeding single bit cell interval and in the direction of the succeeding two bit cell interval. Similar peak phase shifts occur at the succeeding intervals.
While the problem of unequal peak amplitudes of the transition pulses greatly complicates the design of an automatic gain control network for the read circuitry, amplitude equalization is not critical to the accurate recovery of the data. However, since accurate information retrieval requires that the precise intervals between recording code transitions be preserved to recover the data clock, phase shifts in the peaks of the transition pulses can result in decoding errors. Specifically, if intersymbol cross talk results in the phase of certain transition pulse peaks being shifted more than one quarter of a bit cell (0.5 microseconds), errors in the recognition of the timing sequence of the recording code transitions, and thereby errors in the detection of the information contained in the peak phases of these transitions, and the information may be difficult to recover. Furthermore, if the peaks are shifted a significant fraction of 0.5 microseconds, the decoding process becomes significantly more susceptible to noise.
In order to achieve a reliable double density recording capability, various techniques have been used to counteract the intersymbol cross talk caused by the above described superposition effects and restore the proper phase of the transition pulse peaks. For instance, it has been suggested that a tapped delay line be utilized to obtain a number of time-delayed read signals, which are then scaled and summed in a manner to narrow the width of the Gaussian-shaped transition pulses of the read signal, thereby reducing the superposition effect of each transition pulse on the adjacent transition pulses. This technique has the disadvantage that, since it uses time-delayed reproductions of the complete read signal, including superimposed high frequency noise, the summing process tends to enhance the noise signals to the extent that the reliability of the technique is severly limited.
The most prevalent approach to the peak phase shifting problem of intersymbol cross talk uses phase locked loop (variable frequency oscillator) data separation and write pre-compensation. The VFO, once sychronized, tracks the data and generates clock and data windows improving bit shift tolerance. Simultaneously, write pre-compensation circuitry is used to provide peak shift correction by introducing a compensating phase shift as the data are being recorded onto the disc. That is, the phase of the transitions of the write signal waveform are shifted in a direction opposite to the peak phase shift which is introduced by reason of the transfer characteristics of the recording medium and the nonsymmetrical superposition of the Gaussian-shaped transition pulses of the read signal. Thus, if a certain data pattern would cause a particular transition pulse peak to be shifted to a later position in time than nominal, the write pre-compensation circuit causes the associated coded transition to be recorded on the recording medium earlier in time than nominal, thereby compensating for the unavoidable distortion that results from the read process.
The combined VFO and write pre-compensation technique has a number of disadvantages. First of all, because write pre-compensation results in the transition pulses of the read signal being crowded together more than is the case without pre-compensation, the signal-to-noise ratio of the read signal is significantly degraded. Secondly, the crowding together of the Gaussian-shaped transition pulses, and the resulting increase in superposition effects, causes a further disparity in the peak amplitudes of the transition pulses. Finally, the amount of write pre-compensation to be used in recording the digital data onto the recording medium must be decided beforehand, with the result of this decision being permanently recorded on the disc. Inasmuch as the several manufactures of disc recording equipment do not have a general agreement on how much write pre-compensation is to be used, the interchangeability of discs and disc drives is severely limited.
It is a general object of the present invention to provide a pulse shaping network that substantially reduces intersymbol cross talk between the bivalent transition pulses derived from the process of reading information recorded in a high density format on a moving recording medium. To this end, a specific object of the present invention is to provide such a pulse shaping network that corrects peak phase shift and peak amplitude distortion by processing the read signal, without the necessity of altering the manner in which digital data is written onto the recording medium.
A further object of the present invention is to provide a pulse shaping network that can be readily adapted to effecting peak phase shift and peak amplitude distortion corrections for a variety of recording media, each exhibiting unique transfer characteristics.
Another object of the present invention is to provide a band width-limited pulse shaping network that significantly reduces intersymbol cross talk while avoiding significant degredation of the signal-to-noise ratio.
Still another object of the present invention is to provide such a pulse shaping network whose response to a particular read signal can be readily adjusted with only minor component alterations and/or additions in order to optimize read signal correction.